# Lecture 6c. Introduction Electromagnetic spectrum Visible range: =380-750 nm Ultraviolet: =190-380 nm Low energyHigh energy

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Lecture 6c

Lecture 6cUV-Vis SpectroscopyIntroductionElectromagnetic spectrum

Visible range: l=380-750 nmUltraviolet: l=190-380 nm

Low energyHigh energyElectronic TransitionsMost molecules absorb electromagnetic radiation in the visible and/or the ultraviolet rangeThe absorption of electromagnetic radiation causes electrons to be excited, which results in a promotion of an electron from a bonding (s or p) or non-bonding orbital (n) to an anti-bonding orbital (s* or p*) The larger the energy gap is, the higher the frequency and the shorter the wavelength of the radiation required is (h= Plancks constant)Allowed transitions i.e., s-s*, p-p* are usually strong (large e), while forbiddentransitions (low e) i.e., n-s*, n-p* aremuch weaker compared to theseMany transition metal compounds are colored because the d-d transitions fall in the visible range (note that the d-orbitals are not shown to keep the diagram simple)

h= 6.626*10-34 J*sc= 3.00*108 m/s3Color WheelWhen determining a color, one has to know if the process that causes the color is due to emission or due to absorption of electromagnetic radiationExample 1: Sodium atoms emit light at l=589 nm resulting in a yellow-orange flameExample 2: Indigo absorbs light at l=605 nm which is in the orange range the compound assumes the complementary color (blue-purple)

What determines the Wavelength?Most simple alkenes and ketones absorb in the UV-range because the p-p* and the n-p* energy gaps are quite largeConjugation causes a bathochromic shift (red shift)Increased conjugation often also increases the peak size as well (hyperchromic)

Compoundlmax(nm)e(cm-1*mol-1*L)Chromophore1,4-Pentadiene178 26000isolated C=C2-Pentanone180 900isolated C=Ob-Carotene480133000conjugated C=C3-Pentenone224 12590conjugated C=OAcetophenone246 9800conjugated C=O

The p-p* energy gap for the C=C bond is largeThe p-p* and the n-p* energy gap in a C=O bond are bothrelatively large as wellThe combination of these two groups affords a new orbital set in which n-p* and the p-p* gaps are much smaller compared in the isolated bondsIf less energy is required to excite the electrons, a shift tohigher wavelengths for the excitation will be observedi.e., l(n-p*) > l(p-p*)

Conjugation

Bottom line: The exact peak location (l) and absolute peak intensity (e) depend to a certain degree on the solvent used in the measurement

Solventl(nm) eMethanol500 1120331 646025824500Dioxane5041410332708026026000Cyclohexane5121320335710026227100

300 nm600 nmp-p*330 nmn-p*500 nmBeer Lambert Law IIt describes the attenuation of electromagnetic radiation

The cell dimension (l) is usually 1 cm The e-value is wavelength dependent a spectrum is a plot of the e-values as the function of the wavelengthThe larger the e-value is, the larger the peak is going to beThe data given in the literature only list the wavelengths and e-values (or its log value) of the peak maxima i.e., 331 (6460)The desirable concentration of the sample is determined by the largest and smallest e-values of the peaks in the spectral window to be measured

Beer Lambert Law IIThe absorbance readings for the sample have to be in the range from Amin=0.1 and Amax=1 in order to be reliableThe concentration limitations are due to Association at higher concentrations (c>10-4 M)Linear response of the detector in the UV-spectrophotometer

Linear rangeConcentrationAbsorbance0.11.0cmincmaxPractical Aspects of UV-Vis ICuvetteIt cannot absorb in the measurement window Plastic cuvettes absorb more or less in the UV-range alreadyMost test tubes (borosilicates) start to absorb around 340 nmQuartz cuvettes have a larger optical window but are very expensive (>\$100 each)It has to be stable towards the solvent and the compoundMost plastic cuvettes are etched or dissolved by low polarity solvents and can only be used with alcohols or waterQuartz cuvettes are stable when used with most organic solvents

PolystyrenePolymethacrylateQuartz

detector

PolyethylenecuvettelampPractical Aspects of UV-Vis IISolvent

Hydrocarbons and alcohols possess the largest optical windows Note that spectrograde solvents should be used whenever possible because many non-spectrograde solvents contain additives i.e., 95 % ethanol contains a lot of aromatics that are active in the UV range

Solventlower limit (l in nm)Absorbance for l=1 cmAcetone330335 (0.30), 340 (0.08), 350 (0.003)Acetonitrile190200 (0.10), 210 (0.046), 230 (0.009)Chloroform265250 (0.40), 260 (0.05), 270 (0.006)Cyclohexane210210 (0.70), 220 (0.32), 230 (0.11), 240 (0.04)Dichloromethane235230 (1.30), 240 (0.15), 250 (0.02)Ethanol (abs.)210210 (0.70), 220 (0.4), 240 (0.1), 260 (0.009)Hexane210210 (0.30), 220 (0.1), 230 (0.03), 240 (0.016)Methanol210220 (0.22), 230 (0.1), 240 (0.046), 250 (0.02)Water191Practical Aspects of UV-Vis IIIImportant PointersSince most measurements require a serial dilution, it is imperative that the entire compound is dissolved when preparing the stock solutionFor the calculation of the new concentration, the student needs to keep in mind that the total volume is important i.e., if 1 mL of the stock solution was used and 9 mL of additional solvent, the concentration is one tenth of the original concentrationThe student has to run a full spectrum, which requires the software to be set to spectrum mode and not to fixed wavelength mode (see pop down window in the upper left hand corner)

Practical Aspects of UV-Vis IVUV-Vis detector is used in HPLCThe chromatogram changes significantly as the wavelength is changed because compounds display different e-values at a given wavelengthThe different absorbance characteristics can be used to detector specific compounds and other not The linear range for quantitation is limited and given by the wavelength chosen for quantitation

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